Polar ordering of reactive chromophores in layer-by-layer nonlinear optical materials

ABSTRACT

Thin films exhibiting second-order nonlinear optical (NLO) properties, as well as materials and methods for producing such films, are provided. The films are formed by depositing, on a substrate, alternate layers of a polyelectrolyte and a low molecular weight chromophore. The chromophore contains an electrophilic group that reacts with a previously deposited polyelectrolyte, and ionizable groups which present absorption sites for the next polyelectrolyte layer. The films find application in, for example, electro-optic modulators and frequency doubling devices.

This application claims benefit to Provisional application Ser. No.60/330,907, filed Nov. 2, 2001.

This invention was made using funds from grants from the NationalScience Foundation having grant number ECS-9907747. The government mayhave certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to thin films that exhibit second-ordernonlinear optical (NLO) properties. In particular, the inventionprovides materials and methods for producing such films by forming, on asubstrate, alternate layers of a polymer and a low molecular weightchromophore.

2. Background of the Invention

Materials that exhibit second-order nonlinear optical (NLO) propertiesare key components in electrooptic modulators and frequency-doublingdevices. [1] Second-harmonic generation (SHG), in which incident lightat one frequency is converted into light at twice that frequency, is oneexample of second-order NLO phenomena and is often used as anexperimental probe of the second-order susceptibility χ⁽²⁾. A materialmust have a non-centrosymmetric structure to possess a nonzero χ⁽²⁾.Electrooptic modulators have traditionally employed ferroelectricinorganic crystals, such as lithium niobate or potassium dihydrogenphosphate, which are formed at high temperatures. However, organic NLOmaterials offer several advantages in performance, such as highernonlinear susceptibilities, higher modulation rates, and potentiallylower device fabrication costs. [2] Organic films exhibiting nonzeroχ⁽²⁾ values have been fabricated using a variety of methods, includingelectric field poling, [3] Langmuir-Blodgett (LB) films, [4] andcovalent self-assembly. [5] Both poled polymer systems and LB films havebeen made with non-centrosymmetric structures that exhibit relativelyhigh values for χ⁽²⁾, but poor temporal or mechanical stability restricttheir potential applications. [6] Deposition processes using reactivesilane compounds require organic solvents and high temperatures. [5]

There is a large and growing body of literature on the use oflayer-by-layer (LBL) methods for fabricating nanostructured films for avariety of applications. The LBL technique, which relies on purelyelectrostatic interactions, was first developed by Iler [7] and furtherelaborated upon by Decher et al. [8] Several research groups havedemonstrated that the NLO films made by this technique have greaterthermal and temporal stability than poled polymer systems. [9] A relatedapproach that could be employed to fabricate NLO materials involves theuse of low-molecular-weight dye molecules and polymers as filmconstituents. Yamada et al. made films of poly(diallyldimethylammoniumchloride) and Erichrome Black T that exhibited an SHG intensity thatincreased only for the first five bilayers and then reached a plateau.[10] Koetse et al. [18] experimented with films grown with polyaminesand reactive dyes but found that there was no polar order within thefilm layers. The authors indicate that the SHG signal they observed mostlikely originated from the dye molecules at the surface of the support.Other research groups have found that ionic interactions alone are notsufficient for constructing LBL films with low-molecular-weightchromophores. [11]

It would be of great benefit to have available stable NLO films with thelarge number of bilayers needed for electrooptic devices, and methodsfor their production. Further, it would be of benefit to have availablea combination of low-molecular-weight chromophoric molecules andpolymers that could be used to construct such films.

SUMMARY OF THE INVENTION

The present invention provides new materials and new methods to producemultilayer films with high second-order nonlinear optical (NLO)susceptibility (χ⁽²⁾) values.

In one embodiment, the invention provides a method of forming amultilayered nonlinear optical material. The method includes the stepsof adsorbing a first species to the surface of a substrate to form afirst species layer; attaching a chromophore to the first species layerto form a first chromophore layer; adsorbing the first species to thefirst chromophore layer to form a repeat first species layer; andattaching the chromophore to the repeat first species layer to form arepeat chromophore layer, and repeating this process multiple times.Alternatively, the order of deposition may be reversed, depending on thechemical composition of the components. In one embodiment, the firstspecies is a polymer such as poly(allylamine hydrochloride) (PAH),polyvinylamine (PVA), poly-(l-lysine) (PLL), or poly(ethylene-imine)(PEI). In another embodiment, the first species is an organosilane or analkanethiol compound. The adsorbing step may be performed underconditions whereby a covalent bond or a non-covalent bond (e.g. an ionicbond) is formed between the first species and the substrate. The methodmay further include the step of controlling the density of the firstlayer formed during the adsorbing step. The attaching step for thechromophore may be performed by forming a covalent bond between thechromophore and the first species of the first layer.

In a preferred embodiment, the chromophore is of a molecular weightranging from about 100 to about 2000 atomic mass units, and thechromophore may be a monomer, for example, Procion Red MX5B, ProcionBrown MX-GRN, or Procion Orange MX-2R, or a trichloro-s-triazinecovalently bonded to a dye such as Mordant Brown 33, Acid Red 37, andDirect Orange 31. The first chromophore layer may exhibitnoncentrosymmetry.

The method may further comprise the step of repeating the adsorbing andattaching steps multiple times. In addition, the first species in thefirst layer may be chemically or physically different from the firstspecies in the repeat first species layer, and the first species in atleast two of the repeat first species layers may be chemically orphysically different from one another. The chromophore in the firstchromophore layer may be chemically or physically different from thechromophore in the repeat chromophore layer, and the chromophore in atleast two of the repeat chromophore layers may be chemically orphysically different from one another.

The steps of attaching and adsorbing may be accomplished by submersionof the substrate into, respectively, a volume of the first species and avolume of the chromophore. Alternatively, the attaching and adsorbingsteps may be accomplished by washing a solution of the first species orthe chromophore over the substrate. The method may further comprise thestep of removing the substrate after all steps are completed.

The invention further provides a film with high second-order nonlinearoptical susceptibility. The film includes alternating layers of apolymer or a self-assembled monomer, and a low molecular weightchromophore. The alternating layers may be attached to a solid substratethrough deposition of a first layer of the polymer or the self-assembledmonomer onto the solid substrate. The solid substrate may comprise ionicgroups or groups that can react with a self-assembled monolayer, and thedeposition may occur through covalent or non-covalent self-assembly. Theionic groups may be negatively charged and the charged solid substratemay be comprised of material such as glass and silica.

The polymer or the self-assembled monomer may comprise ionic groupswhich are also nucleophilic, depending on the pH. The polymer may be anamine containing polymer such as poly(allylamine hydrochloride) (PAH),polyvinylamine (PVA), poly-(l-lysine) (PLL), or poly(ethylene-imine)(PEI).

The low molecular weight chromophore may be a monomer, and may comprisechemical moieties capable of covalently bonding to the polymer or theself-assembled monomer, and chemical moieties capable of bindingionically to the polymer or the self-assembled monomer. Such chemicalmoieties may be electrophilic moieties such as those with one or moretriazine rings. The ionic moieties may be ionizable groups such assulfonate, phosphate, carboxylate, or quaternary ammonium.

The film may display noncentrosymmetric order.

The invention also provides a second order NLO material comprisinglayers of a chromophore and layers of a polymer. The layers are heldtogether by alternating covalent and non-covalent bonding betweenlayers. The non-covalent bonds may be ionic, hydrophobic, or hydrogenbonds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Schematic representation of NLO thin film. 10=substrate; 20a=first polymer layer; 30 a=first chromophore layer; 20 b=second polymerlayer; 30 b=second chromophore layer.

FIGS. 2A, B, and C. Molecular formula of A, Mordant Brown 33; B, AcidRed 37; and C, Direct Orange 31.

FIG. 3. Novel triazine-containing chromophore. R=Cl or O-alkyl.

FIGS. 4A and B. A: depiction of polyallylamine hydrochloride (PAH) andProcion Red dye (PR). B: Schematic of PAH ionically bonded to asubstrate, and covalent attachment of PR to the PAH layer.

FIGS. 5A, B and C. A: Absorbance (A) at 538 nm as a function of thenumber of bilayers (n) in PR-PAH films. Experimental conditions for A-Dcorrespond to entries in Table 1. Lines are obtained by linearregression analysis. ⋄=PAH pH 4.5/PR pH 7.0; □=PAH pH 4.5/PR pH 10.5;Δ=PAH pH 7.0/PR pH 7.0; ◯=PAH pH 7.0/PR pH 10.5. B: In situ SHGmeasurement of the reactive coupling of PR onto PAH. C: Square root ofthe SHG intensity (I_(2ω))^(1/2) as a function of the number of bilayersfor films with experimental conditions for A-D corresponding to Table 1.Lines are obtained by linear regression analysis. ⋄=PAH pH 4.5/PR pH7.0; □=PAH pH 4.5/PR pH 10.5; Δ=PAH pH 7.0/PR pH 7.0; ◯=PAH pH 7.0/PR pH10.5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Applicants have discovered new materials and new methods to producemultilayer films with high second-order nonlinear optical (NLO)susceptibility (χ⁽²⁾ [19]) values. The films comprise alternating layersof a polymer and a monomeric chromophore. The polymer is deposited fromsolution onto the surface of a charged solid substrate, for example, byan ionic self assembly technique. The chromophore is then attached(e.g., covalently) to the immobilized polymer layer, forming an outerchromophore layer to which a subsequent polymer layer is then attached,and so on. By alternating the methods of deposition for each monolayer(e.g., covalent reaction alternated with electrostatic interaction) anddecoupling the chromophore orientation from the steric constraints of apolymer chain (or high molecular weight and “bulky” sterochemistry ingeneral), the non-centrosymmetric orientation required for nonzero χ⁽²⁾values is achieved. The films thus formed possess high net polarordering in each bilayer, and are useful for applications requiringfilms which display noncentrosymmetric order (e.g., variouselectrooptical applications).

A schematic representation of the film is given in FIG. 1, where asubstrate 10 is depicted. A first layer of a polymer 20 a is ionicallyattached to charged functional groups of the substrate. Next, a firstlayer 30 a of a chromophore is covalently linked to the polymer layer, asecond polymer layer 20 b is ionically bonded to functional groups ofthe chromophore layer 30 a, a second chromophore layer 30 b is attachedcovalently to the second polymer layer 20 b, and so on. The order ofdeposition may be reversed, depending on the chemical nature of thecomponents and the substrate (i.e., chromophore-polymer-chromophore).

One indicator of high NLO is the level of second harmonic generation(SHG) exhibited by the film, as measured by the intensity of lightgenerated at a wavelength which is one-half that of light incident onthe film. In general, a film with “high” NLO will exhibit SHG χ⁽²⁾ valuelarger than about 5×10⁻⁹ esu. Most preferably, the esu value will be inexcess of at least about 30×10⁻⁹ esu.

The methods involve providing a substrate onto which the film will belayered. Examples of suitable substrates include but are not limited toglass, silica, silicon wafers, indium tin oxide, other metal oxidesurfaces, gold, other metal surfaces, polymer surfaces with chargedgroups such as: (1) carboxylic acids that form when a polymer surface islightly oxidized, and (2) sulfonate groups that form on polystyrenesurfaces that are subjected to a sulfonation reaction, etc. Those ofskill in the art will recognize that substrates made of such materialswill possess surface accessible functional groups to which complimentarygroups of the polymer molecules (i.e., chemical moieties of the polymercapable of bonding with the functional groups of the substrate) willadsorb, i.e., will be physically or chemically attached. In a preferredembodiment, the functional groups of the substrate and the complementaryfunctional groups of the polymer bear a charge and bond ionically. Forexample, the substrate may have surface accessible functional groupsbearing a negative charge and the polymer may contain complementarygroups which bear a positive charge. Alternatively, the substrate mayhave surface accessible functional groups bearing a positive charge andthe polymer may contain complementary groups which bear a negativecharge.

Those of skill in the art will recognize that the substrates on whichthe films are layered may be of many different forms, shapes and sizes,as suitable for a particular application. Examples include but are notlimited to planar (flat), curved, spherical, irregular, circular,rectangular, cylindrical, and the like. For example, a planar substrategeometry is often used in fabricating electro-optic modulators. Thesubstrate may be of any desired morphology, so long as it is of amaterial to which the initial polymer layer can be physically orchemically attached, and to which subsequent layers of chromophore andpolymer can be added. Further, the polymer need not attach to the entiresubstrate (i.e., in some applications it may be preferable to build thelayered structure on only a limited area of the substrate).Alternatively, more than one polymer may be bonded to the substrate(i.e., a mixture of polymers may be used, either concomitantly or oneafter the other) or different polymers may be used on different sectionsof the substrate to form a “pattern” of layers of the same or differentcompositions on the substrate, or different layers of a single film maybe formed from different polymers and chromophores. Further, thesubstrate itself may be designed to be comprised of different materialsin different areas, for example, to promote the binding of one type ofelectrolyte to one area, and a different electrolyte to another area,resulting in a pattern of films of different compositions on differentareas of the substrate. The areas may be adjacent, or they may be spacedapart by areas with no NLO film. In addition, the film may be depositedon one or several surfaces of the substrate, e.g., on a front, back andsides of a planar substrate, or only on one or more of those surfaces.The film may also be deposited on one substrate and utilized in anapplication intact (i.e., the substrate and film constitute a unit whichis utilized). Alternatively, the film may be removed from the originalsubstrate on which it was deposited and utilized in an applicationwithout the substrate, or on another substrate. In preferredembodiments, such as that of an electro-optical modulator, the film isretained on the original substrate on which it was formed.

The first layer of the film is comprised of a polymer or other chemicalspecies which is adsorbed to the substrate. The components in this layercould be polymers, or they could be smaller molecules, such asorganosilane or alkanethiol compounds. Examples of positively chargedpolymers adsorbed to a negatively charged substrate (such as glass)include but are not limited to poly-(allylamine hydrochloride) (PAH),polyvinylamine (PVA), poly-(l-lysine) (PLL), and poly(ethylene-imine)(PEI). In a preferred embodiment, the attachment of the polymer to thesubstrate is via ionic bonding. Another example using monomericmolecules as components of the first layer would include, but not belimited to, organosilane reagents such as 3-aminopropyltrimethoxysilane.The adsorbed amount of polymer should be in the range of about 0.5milligram adsorbed polymer per square meter of surface or higher, whichcorresponds roughly to one monolayer of adsorbed polymer. The density ofthe first layer may be controlled by any of several means, e.g., byadjusting the concentration of the polymer solution, the pH, the ionicstrength, the time of exposure to the solution, the temperature, etc.Further, the density need not be the same for all layers. For example,the initial layer attached to the substrate may be of one density,whereas for a particular application, it may be preferable forsubsequent polymer layers (those deposited on chromophore layers) to bemore or less densely applied.

In order to form the multilayered film, a layer of chromophore moleculesis attached to the first polymer layer. The chromophore is a moleculewith a system of conjugated pi-bonds and contains two moieties thatcontribute to the layer-by-layer building of the film and thenoncentrosymmetry: 1) chemical moieties capable of covalently bonding tofunctional groups on the underlying polymer layer and 2) chemical groups(e.g., ionizable groups) that present adsorption sites for the nextpolymer layer. In a preferred embodiment, the chromophore is depositedon the polymer layer by a covalent bonding reaction with suitablefunctional groups of the previously deposited polymer layer. Forexample, electrophilic groups contributed by the chromophore may form acovalent bond with nucleophilic groups of the polymer layer, ornucleophilic groups contributed by the chromophore may form a covalentbond with electrophilic groups of the polymer layer. The pH,temperature, or other properties of the deposition solutions may be usedto control the reactivity between the nucleophilic and electrophilicspecies. Activating reagents may also be added to enhance the reactivitybetween the chromophore and polymer. Such covalent attachment serves toconfer stability to the film and orientation of the chromophore.

If a reactive chromophore such as Procion Red MX5B is deposited bynon-covalent bonding, there is very little polar ordering of thechromophore. The choice of the polymer is also important in conferringpolar ordering on the chromophore. A polymer such as PAH confers polarordering of Procian red MX5B, while a polymer such as PVBDA (Koeste etal. [18]) does not. Examples of groups which are suitable for covalentbonding in this manner include but are not limited to: for the polymer,amine, hydroxyl, sulfhydryl, carboxylate, aldehyde, epoxy, hydrazide,etc; and for the chromophore, mono- or di-chlorotriazine, vinyl sulfone,epoxy, carboxylate (after activation by a carbodiimide), amine,aldehyde, hydrazide, etc.

While in a preferred embodiment of the invention, the mode of attachmentof the chromophore is covalent, other alternating adsorption mechanismsleading to chromophore orientation are possible, for example, mechanismsinvolving adsorption driven by ionic and hydrogen bonding interactions.As long as one functional group of the chromophore interacts with thepolymer layer more strongly than another functional group of thechromophore, orientation of the chromophore on the surface onto which itis being deposited will result. For example, a chromophore with twodifferent ionic groups each located at opposite “ends” of thechromophore (i.e., at distal portions of the molecule) could be absorbedwith a preferred orientation due to the differing electrostaticinteractions of the two groups with an oppositely charged surface suchas that presented by the polymer layer. For example, one ionic groupcould be designed to interact with charged moieties on the polymer layerunder one set of conditions (e.g. a certain pH) whereas the second ionicgroup would remain uncharged during deposition onto the polymer layer.After deposition, the second ionic group is, by virtue of being locatedon the “end” of the chromophore opposite to that which is bound to thepolymer layer, oriented away from the layer and is relatively exposed.Conditions could then be altered to activate the second ionic group(e.g., a change in pH that causes ionization of that group) which wouldbecome capable of adsorbing the next layer of polymer.

In a preferred embodiment, the chemical groups of the chromophore thatpresent adsorption sites for the next polymer layer are ionizable groupswhich, upon ionization, can undergo non-covalent bonding (for example,ionic bonding or hydrogen bonding) with suitable functional groups of anext polymer layer. Examples of such ionizable groups include but arenot limited to sulfonate, phosphate, carboxylate, quaternary ammonium,etc. Such groups could bond with, for example, suitable charged groupslocated on an incoming polymer layer (e.g., a chromophore-sulfonatecould interact with a polymer-quaternary ammonium).

In preferred embodiments, the chromophore which is utilized is a lowmolecular weight chromophore. By “low molecular weight chromophore” wemean a chromophore molecule with a molecular weight in the range ofabout 100 to about 2000 Atomic Mass Units (AMU). In some preferredembodiments, the chromophore is monomeric, i.e., the molecule contains asingle repeat unit and is not linked to other chromophores by a polymerbackbone. The purpose of utilizing a low molecular weight or monomericchromophore is to allow greater flexibility with respect to the amountof orientation that can be introduced into the layers. When polymersthat contain chromophores in the form of bulky side chains are utilizedin the formation of thin film layers, steric hindrance puts limits onthe amount of orientation that can be achieved. In contrast, lowmolecular weight chromophores (e.g., monomeric chromophores) do not haveinherent steric hindrance to hinder attainment of a desired orientation.Examples of suitable low molecular weight chromophores that may beutilized in the practice of the present invention, include but are notlimited to chromophores containing one or more triazine moieties, (e.g.,Procion Red MX5B (CAS registry 17804-49-8), Procion Brown MX-GRN (CAS68892-31-9), and Procion Orange MX-2R (CAS 70616-09-9), etc.

The present invention also provides new chromophores which fulfill thecriterion of the chromophores described herein. In the novelchromophores, moieties capable of binding chemically to an existingpolymer layer (e.g. electrophilic triazine moieties) and functionalgroups capable of providing adsorption sites for a subsequent layer ofpolymer (e.g., ionizable groups) are arranged to obtain a desiredmolecular size and shape, as well as to obtain a desired orientation ofthe reactive functional groups (e.g., the triazine and ionizable groups)for 1) attaching to the existing polymer layer and 2) receiving andbonding to an incoming polymer layer. In addition, the chromophores aredesigned so that the layer which they form displays sufficientnoncentrosymmetry so that the film which is formed will exhibit a highlevel of NLO activity and stability adequate for desired applications.These include triazine derivatives synthesized from amino- or hydroxy-functional water-soluble dyes as outlined in Scheme 2.

Examples of prototype systems are Mordant Brown 33, Acid 37 and DirectOrange 31, all of which are commercially available (CI 13250, 17045, and23655, respectively) (FIGS. 2A-C). Based on known NLO-molecularstructure paradigms (i.e., long conjugation length and electron-donatingand electron-withdrawing groups at opposing ends of the molecule) [1,2]new dyes may be synthesized with the requisite functionalities,including the optimal number and location of sulfonic acid groups, tomaximize the NLO effect. A proposed monosulfonate example a is shown inFIG. 3, the parent molecule b of which has a βμ value of 60,000×10⁻⁴⁸esu [3], 75 times that of Disperse Red I.

The density of chromophore deposition may be controlled by any ofseveral means, e.g., by adjusting the concentration of the chromophoresolution, the density of reacting groups in the underlying polymerlayer, the time of exposure to the chromophore solution, the temperatureof the reaction, the ionic strength and pH of the chromophore solution,etc. Further, the density need not be the same for all chromophorelayers in the multilayered film. The density or concentration ofchromophore in a layer of a film or in the film as a whole may bedetermined by measuring the absorbance of the film in the UV or visibleregion at a wavelength characteristic of the chromophore.

In a preferred embodiment of the invention, deposition of the moleculeof interest (i.e. the polymer or the chromophore) is accomplished byexposure of the substrate to an aqueous solution containing the moleculeto be deposited. However, those of skill in the art will recognize thatthe solution need not be strictly aqueous. Solvent systems containingorganic modifiers (such as, but not limited to, surfactants,cyclodextrins, or solvents such as methanol, acetonitrile, etc.) mayalso be used to form solutions of the polymer or the chromophore, solong as the solution effectively solubilizes the molecule of interest,makes the molecule available for deposition on the substrate (and/or onprevious layers), and does not have deleterious interactions with themolecules of interest or the substrate. In general, the concentration ofthe polymer in such a solution will be preferably in the range of about0.01 to about 0.1 Mole of repeat unit/liter; and the concentration ofthe chromophore in such a solution will be in the range of about 0.1 toabout 10 weight percent, and preferably in the range of about 1 to about5 weight percent. In addition, the solutions containing the moleculesfor deposition may contain other substances of benefit to the depositionprocess, such as salts or organic modifiers. Further, the pH of thesolutions may be adjusted so as to optimize the deposition procedureand/or the chemical reaction between the functional groups of thesubstrate, polymer, or chromophore. For example, if the functional groupof a chromophore that bonds to suitable chemical groups of an incomingpolymer is an ionizable amine, the pH of the solution may be adjusted toensure that most of the amine groups are protonated, thus promotingreaction between the ionized group and the polymer.

Those of skill in the art will recognize that many techniques forcarrying out exposure of the substrate (or substrate plus additionallayers) to the polymer exist and are well known. Examples include butare not limited to: submersion of the substrate into a liquid solutionof the molecule of interest, washing a solution of the molecule over thesurface of the substrate, applying a solution of the molecule byspraying, submersion of the substrate into an electrochemical cellcontaining a liquid solution of the polymer and using an electricalfield to help drive deposition, applying drops of the solution to aspinning substrate (i.e., spin-casting), and the like. Any method ofexposing the substrate to solutions containing a molecule of interest(the molecule to be deposited) may be utilized, so long as the processresults in sufficient deposition of the desired molecule.

Further, the precise mechanics of handling the deposition process (e.g.,exposure of a substrate to a solution of the molecule of interest,washing or rinsing and/or drying or curing of the substrate plusattached layers between steps of deposition or after completion ofdeposition, etc.) may vary from case to case, due to, for example, theidentity of the polymer and the chromophore and the intended applicationof the device in which the film is to be employed, the nature of thesubstrate (e.g., size and shape) on which the film is built up, thescale of the operation in which the films are formed, thickness of thefilm, and the like. Information regarding suitable techniques fordeposition procedures are readily available and well-known to those ofskill in the art. For example, see references [20], [21] and [22].

In preferred embodiments, the layering process of the present inventionis carried out at ambient temperature e.g., at “room temperature”(roughly 25° C.). However, those of skill in the art will recognize thatthe deposition process may be carried out at a wider range oftemperatures, preferably at from about 4 to about 80 degrees.

The time for carrying out the deposition of an individual layer may varyfrom formulation to formulation, depending on, for example, the molecule(e.g., polymer or chromophore) being deposited, the composition of thesolution from which deposition occurs, the nature of the substrate, thedesired density of deposition, and the like. Such factors arewell-understood by those of skill in the art. For example, seereferences [20], [21] and [22]. However, in general, the time requiredfor deposition will be on the order of about 1 to about 60 minutes, andpreferably from about 2 to about 5 minutes. In the examples providedbelow, chromophore deposition was complete within about 2-3 minutes.

The desired quantity or density of a molecule of interest to bedeposited in an individual layer may vary from case to case, dependingon, for example, the intended application of the film; the nature of thepolymers, chromophores and substrates, etc. In general, the amount ofpolymer deposited may vary from about 0.5 to about 5 milligram ofdeposited polymer per square meter of substrate area.

The steps of depositing a layer of polymer (such as a polyelectrolyte),then depositing a layer of chromophore onto the polymer layer, thendepositing another layer of polymer onto the chromophore layer, etc.,can continue as described until the desired thickness of number oflayers is attained. The number of layers to be deposited and the finalthickness of the film may vary from situation to situation. In general,the number of layers will be in the range of about 2 to about 500, andthe thicknesses will range from about 0.5 nm/bilayer to about 10nm/bilayer. For example, for use in an electro-optic modulator, a filmof about 1 to about 10 microns is desirable. In a preferred embodimentof the present invention, the bilayer thickness is about 0.5 nm.

The films of the present invention may be utilized in any of a varietyof electrooptical applications, including but not limited toelectro-optic modulators, switches, directional couplers, andpiezoelectric and pyroelectric devices, and the like. In suchapplications, the required strength of electrical signals could bedecreased. Poled polymer devices have been demonstrated with completemodulation at less than 1 volt (this voltage is often referred to as“V_(π)” and generally it is desired to make it as small as possible,preferably less than 1 volt) applied to the device at speeds greaterthan 100 GHz. However, such devices suffer from stability problems whichhave been overcome by the films of the present invention.

The invention also provides a second order NLO material formed of layersof a chromophore and layers of a polymer. The layers are joined togetherby alternating covalent and non-covalent bonding between layers. Typesof non-covalent bonding are ionic, hydrophobic, and hydrogen bonding.The material may be formed on a substrate, and, depending on theparticular type of material that is being formed, the first layer thatis deposited on the substrate may be a polymer layer, or the first layerthat is deposited on the substrate may be a chromophore layer. Thenature of potential substrates, polymers and chromophores is asdiscussed above.

A further type of second order NLO material is one in which the layersare formed from alternating layers of chromophore and polymer. However,in this material the layers are joined together by ionic bonding. Thechromophore contains two different types of ionic functional groups, oneof which is un-ionized (masked) under a condition in which the other isionized (nucleophilic). To form such a material, the polymer isdeposited on a substrate, and the resulting polymer layer is exposed toand allowed to react with the chromophore under conditions in which oneionic group of the chromophore is ionized and the other ionic group isnot. For example, the reaction may be carried out at a pH at which oneionic functional group is protonated (and therefore charged) but atwhich a second ionic functional group is not protonated and is thereforeneutral. The reaction is allowed to proceed and the chromophore isdeposited on the polymer layer via ionic bonding between reactive ionicgroups of the polymer and the reactive ionic groups of the chromophore.A chromophore layer is thus formed. The deposited chromophore layer isthen exposed to a polymer solution under conditions in which thepreviously un-ionized (masked) ionic functional groups of thechromophore are ionized (unmasked) e.g. by changing the pH to effectprotonation of the unprotonated ionic functional groups. The unmaskedionic groups then provide sites for adsorption of an additional layer ofpolymer via ionic bonding. Following this, yet another layer ofchromophore (again with both ionized and un-ionized ionic functionalgroups) may be deposited on the outermost polymer layer via ionicbonding between the reactive functional groups of the chromophore andionic groups of the polymeric layer. Conditions are again changed tounmask the un-ionized ionic functional groups of the outermostchromophore layer, which is then reacted with additional polymer, and soon. The process is repeated until the desired number of layers and/orthe desired depth of the layered material is achieved. Alternatively,the first layer (i.e. the layer that is deposited directly on thesubstrate) may be a chromophore layer. In this case, the chromophore isattached to the substrate via ionic bonding with reactive ionic groupsof the chromophore, followed by unmasking of unreactive ionic groups,deposition of a polymer layer, and so on, as described above.

Further description of the invention is found in the foregoingnon-limiting Examples.

EXAMPLES Materials and Methods

Procion Red (PR, Aldrich) and poly(allylamine hydrochloride) (PAH; M_(W)ca.70000, Aldrich) were used as received (see FIG. 4A). Glass microscopeslides (Fisher Scientific) were used as the substrates and prepared byusing the RCA cleaning procedure. [16] Solutions of PAH (10 mm, based onthe monomer) and solutions of PR (25 mm) in deionized water were used inall experiments. The pH values of the solutions were adjusted with HClor NaOH. The immersion time in PAH was 5 min, with the exception of thefirst layer, which was immersed for 10 min. The immersion time in PR was10 min. The substrates were rinsed thoroughly with deionized waterbetween immersions. The slides were dried after every ten bilayers withN₂. For each set of conditions, a series of five slides was made with atotal of 2, 10, 20, 40, and 60 bilayers (counting both sides).Absorbance and SHG measurements took into account the films deposited onboth sides of the substrate, while ellipsometry measurements were madewith films deposited on only one side. Absorption measurements were madewith a Hitachi U-2000 spectrophotometer at a wavelength of 538 nm andwere taken every ten bilayers during the deposition process. Filmthicknesses were measured with a variable-angle spectroscopicellipsometer (J. A. Woolam Ellipsometer VB-200). Ellipsometric data wereobtained from the unfrosted face of the slide, as the scatteringeliminates backside reflections and simplifies data analysis. Theamplitude factor (ψ) and phase factor (Δ), which are related to thecomplex Fresnel coefficients for any given film, [17] were measured forwavelengths from 350 to 1000 nm at 10 nm intervals. This wavelengthrange was repeated for angles ranging from 55 to 75° in 5° intervals.

SHG measurements were performed with a standard setup using a 10-nspulse-width, Q-switched Nd:YAG laser with a fundamental wavelength of1064 nm. The SHG data were averaged over 100 shots per data point, andthe uncertainty in the relative χ⁽²⁾ values is 10%. Typical spot radiusand pulse energy values were 30 μm and 7 mJ pulse⁻¹, respectively. Thefilm was deposited on both sides of the substrate. As a result, as thesample was rotated with respect to the incident beam, the path lengthbetween the films on opposite sides was varied, which led tointerference fringes of the SHG intensity. The sample was rotated from30 to 60° away from normal incidence using a stepper-motor-controlledrotation stage. The χ⁽²⁾ values were determined from the peak of theinterference fringe in the vicinity of 45°. By comparison to Makerfringes in a quartz crystal wedge, the χ⁽²⁾ value of a film was obtainedfrom equation 10 $\begin{matrix}\underset{\_}{\frac{\chi_{film}^{(2)}}{\chi_{q}^{(2)}} = {\frac{2l_{cq}}{\pi\quad l_{film}}\sqrt{\frac{l^{\underset{film}{2\omega}}}{l_{q}^{2\omega}}}}} & 10\end{matrix}$where l_(film) is the total path length through the film, the coherencelength of quartz (l_(c,q)=λ/4(n^(2w)−n^(w)) is 22.4 μm, and the χ⁽²⁾value of quartz is 1.92×10⁻⁹ esu.

Example 1

The model anionic/reactive species used in this study was Procion RedMX-5B (PR), and the NLO-inactive polycation was poly(allylaminehydrochloride) (PAH) The layering of PAH and PR onto a substrate isdepicted schematically in FIG. 4B. The pH values of the dippingsolutions determine the ionization state of the amine moieties on PAH,which affects both the conformation of the polymer upon adsorption andits subsequent reactivity with PR. Efficient electrostatic deposition ofPAH requires that the pH value of the PAH solution is maintained near orbelow the pKa value of the amino group (8.7), [12] where the majority ofthe NH₂ groups will be protonated and available for interaction withnegative charges on the substrate or the PR molecule. The pH value ofthe PR solution must be near or above the pKa value of the PAH aminegroups for efficient covalent attachment of the chromophore to thedichlorotriazine ring at room temperature. [13] In situ measurements ofSHG intensity during PR deposition showed that each monolayer wasdeposited within two minutes at room temperature. Successful film growthwas characterized by a linear increase in the absorbance and thicknesswith the number of bilayers deposited (FIG. 5A and Table 1). The pHvalue of the PAH solutions in these experiments was held at either 4.5or 7.0. The positive charges along the PAH chain strongly repel eachother and are strongly attracted to the negatively charged surface.These two effects lead to the deposition of PAH in layers with a bilayerthickness less than 1 nm. Chromophore deposition was complete within 2to 3 minutes (FIG. 5B).

TABLE I Properties of films made by a hybrid deposition process fromProcion Red (PR) and poly(allylamine hydrochloride) (PAH). PAH PR χ⁽²⁾Rel. Rel. Expt pH pH An_(b) ^(−1[a]) t_(b)[nm]^([b]) [×10⁹ esu]^([c])An_(b) ^(−1[c]) (I_(2ω))^(½)N_(b) ^(−1[c]) A 4.5 7.0 (8.9 ± 0.1) × [d] —— — 10⁻⁴ B 4.5 10.5 (2.0 ± 0.1) × 0.34 ± 11.2 ± 0.1  1.00 1.00 10⁻³ 0.02C 7.0 7.0 (2.3 ± 0.4) × 0.55 ±  1.2 ± 0.01 1.15 0.17 10⁻³ 0.05 D 7.010.5 (3.3 ± 0.3) × 0.52 ± 11.3 ± 0.1  1.63 1.55 10⁻³ 0.06 ^([a])A =absorbance at 538 nm; n_(b) = number of bilayers as determined by linearregression analysis. ^([b])t_(b) = thickness per bilayer as determinedby ellipsometry. ^([c])Relative values of the slopes obtained by linearregression analysis of each series in FIG. 1 and 2 relative toexperiment B. ^([d])Too thin to measure. ^([e])For quartz, χ⁽²⁾ = 1.92 ×10⁻⁹ esu.

The pH value of the PR solution has a significant effect on the amountof chromophore incorporated into the film. When the pH value of the PAHsolution was held constant at 4.5 or 7.0 and that of the PR solution wasincreased from 7.0 to 10.5, the amount of PR deposited per bilayerincreased, which is consistent with the increased covalent-couplingefficiency. When the pH value of the PR solution was 10.5 and that ofthe PAH solution was increased from 4.5 to 7.0 (compare experiments Band D), a statistically insignificant increase in the film thickness perbilayer (p>0.05) was observed. However, this also resulted in a smallbut significant increase in the amount of PR deposited per bilayer(p<0.05), as measured by the absorbance per bilayer. Importantly, the pHvalue of the PR solutions affects the degree of ordering of thechromophore molecules in the film. When the film thickness is much lessthan the SHG coherence length (typically a few microns), the SHG shouldhave a quadratic dependence on the film thickness. The linear dependenceof the square root of the SHG intensity ((I_(2ω))^(1/2)) on the numberof bilayers for the films fabricated with a PR solution at pH 10.5 thusdemonstrates there is equivalent polar ordering in each successivebilayer (FIG. 5C: lines B and D). When the pH value of the PR solutionis held constant at 10.5, an increase in the pH value of the PAHsolution from 4.5 to 7.0 results in an increase in the absorbance perbilayer (FIG. 5A and Table 1, lines B and D). The value of(I_(2ω))^(1/2) per bilayer also increases by a similar relative amountand the χ⁽²⁾ value of the films is similar, which indicates that forthese conditions the increase in SHG is simply a result of theincorporation of more PR. When the pH value of the PAH solution is heldconstant at 7.0 and that of the PR solution changes from 7.0 to 10.5,the bilayer thickness is similar, but the absorbance per bilayer and theχ⁽²⁾ value all increase. While 42% more PR is incorporated per bilayer,the values of (I_(2ω))^(1/2) per bilayer and χ⁽²⁾ increase approximatelyeightfold. The χ⁽²⁾ values for films made with the PR solution at pH10.5 are six times greater than that of quartz.

The dependence of the values of χ⁽²⁾ and (I_(2ω))^(1/2) per bilayer onthe pH value of the PR solution indicates that the mechanism of PRdeposition has a dramatic effect on the orientation of the PR moleculesin the film. For the films made with a PR solution at pH 7.0(experiments A and C), there is a much weaker dependence of the SHGintensity on the number of bilayers and a much lower χ⁽²⁾ value, whichindicates a lower degree of molecular ordering of the PR molecules. AtpH 7, the reactivity of the PAH amine groups with the triazine ring ofthe PR is lower, and PR may be incorporated by a combination ofelectrostatic, hydrogen bonding, and covalent interactions. Thus atconditions favoring a covalent reaction between PR and PAH, a highdegree of ordering of the PR molecules results, while under unfavorableconditions, the PR deposited within the films has a more randomorientation. We interpret the larger χ⁽²⁾ value and SHG observed in thedeposition of PR at pH 10.5 as confirmation of our hypothesis thatalternating the mechanism of deposition in these films and decoupling ofthe chromophore from steric constraints of a polymer backbone provides aroute to low-molecular-weight chromophores. In our earlier studies withpolymeric dyes bearing NLO-active chromophores as charged pendantgroups, films were made under deposition conditions leading torelatively thin bilayers (ca. 0.2 nm). [14] We found by measuring SHGintensity in situ during deposition that the polar ordering of anadsorbed chromophore layer is reduced by subsequent adsorption of thenext polyelectrolyte monolayer. [15] Decoupling the NLO-activechromophore from a polymer chain reduces the steric constraints forordering that are present when using a large chromophoric side group.Additionally, competing intermolecular interactions that prevent theachievement of a high net polar ordering of the chromophore within thefilm can be minimized by using a bifunctional. chromophore with whichthe mechanism for deposition (covalent bonding) differs from thedeposition of the polymeric component (electrostatic interaction).

This example demonstrates new methodology for making organic LBL filmswith non-centrosymmetric ordering of low-molecular-weight chromophoresin each bilayer and significant χ⁽²⁾ values.

While the invention has been described in terms of its preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims. Accordingly, the present invention should not belimited to the embodiments as described above, but should furtherinclude all modifications and equivalents thereof within the spirit andscope of the description provided herein.

References

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1. A method of forming a multilayered nonlinear optical material,comprising the steps of: a) adsorbing a first species to the surface ofa substrate to form a first species layer; b) attaching a chromophore tosaid first species layer to form a first chromophore layer; c) adsorbingsaid first species to said first chromophore layer to form a repeatfirst species layer; and d) attaching said chromophore to said repeatfirst species layer to form a repeat chromophore layer, wherein layersof said multilayered nonlinear optical material are held together byalternating covalent and non-covalent bonding between said layers. 2.The method of claim 1 wherein the first species is a polymer.
 3. Themethod of claim 2 wherein the polymer is selected from the groupconsisting of poly(allylamine hydrochloride) (PAH), polyvinylamine(PVA), poly-(l-lysine) (PLL), and poly(ethylene-imine) (PEI).
 4. Themethod of claim 1 wherein the first species is a monomeric compoundselected from the group consisting of organosilane compounds andalkanethiol compounds.
 5. The method of claim 1 wherein said adsorbingstep a) is performed under conditions whereby an ionic bond is formedbetween said first species and said substrate.
 6. The method of claim 1further comprising the step of controlling the density of said firstspecies layer.
 7. The method of claim 1 wherein said attaching steps areperformed by forming a covalent bond between said chromophore and saidfirst species.
 8. The method of claim 1 wherein said chromophore is of amolecular weight ranging from 100 to 2000 atomic mass units.
 9. Themethod of claim 1 wherein said chromophore is a monomer.
 10. The methodof claim 9 wherein said chromophore is selected from the groupconsisting of Procion Red MX5B, Procion Brown MX-GRN, and Procion OrangeMX-2R.
 11. The method of claim 9 wherein said monomer is atrichloro-s-triazine covalently bonded to a dye.
 12. The method of claim11 wherein said dye is selected from the group consisting of MordantBrown 33, Acid Red 37, and Direct Orange
 31. 13. The method of claim 9wherein said monomeric chromophore comprises i) chemical moietiescapable of covalently bonding to said first species layer or said repeatfirst species layer; and ii) chemical moieties capable of bindingionically to said first species layer or said repeat first specieslayer.
 14. The method of claim 13 wherein said chemical moieties capableof covalently bonding comprise one or more triazine rings.
 15. Themethod of claim 13 wherein said chemical moieties capable of bindingionically are ionizable groups selected from the group consisting ofsulfonate, phosphate, carboxylate, and quaternary ammonium.
 16. Themethod of claim 1 further comprising the step of controlling the densityof said chromophore layer by adjusting the ionic strength of a reactionmixture in which said steps of said method are carried out.
 17. Themethod of claim 1, wherein said non-covalent bonding is selected fromthe group consisting of ionic bonding, hydrophobic bonding, and hydrogenbonding.
 18. The method of claim 1 wherein said first chromophore layerexhibits noncentrosymmetry.
 19. The method of claim 1 further comprisingthe step of repeating steps c) and d) multiple times.
 20. The method ofclaim 1 wherein said first species in said first species layer ischemically or physically different from said first species in saidrepeat first species layer.
 21. The method of claim 19 wherein saidfirst species in at least two of said repeat first species layers arechemically or physically different from one another.
 22. The method ofclaim 1 wherein said chromophore in said first chromophore layer ischemically or physically different from said chromophore in said repeatchromophore layer.
 23. The method of claim 19 wherein said chromophorein at least two of said repeat chromophore layers are chemically orphysically different from one another.
 24. The method of claim 1 whereineach of said attaching and adsorbing steps are accomplished bysubmersion of said substrate into, respectively, a volume of said firstspecies and a volume of said chromophore.
 25. The method of claim 1wherein each of said attaching and adsorbing steps are accomplished bywashing a solution of said first species or said chromophore over saidsubstrate.
 26. The method of claim 1 further comprising the step ofremoving said substrate after steps a)-d) are completed.
 27. A method offorming a multilayered nonlinear optical material, comprising the stepsof: a) attaching a chromophore to the surface of a substrate to form afirst chromophore layer; b) adsorbing a first species to said firstchromophore layer to form a first species layer; c) attaching saidchromophore to said first species layer to form a repeat chromophorelayer; and c) adsorbing said first species to said repeat chromophorelayer to form a repeat first species layer, wherein layers of saidmultilayered nonlinear optical material are held together by alternatingcovalent and non-covalent bonding between said layers and wherein saidstep of attaching said chromophore to said first species layer isperformed by forming a covalent bond between said chromophore and saidfirst species.
 28. The method of claim 27 wherein the first species is apolymer.
 29. The method of claim 28 wherein the polymer is selected fromthe group consisting of poly(allylamine hydrochloride) (PAH),polyvinylamine (PVA), poly-(l-lysine) (PLL), and poly(ethylene-imine)(PEI).
 30. The method of claim 27 wherein said adsorbing step a) isperformed under conditions whereby an ionic or covalent bond is formedbetween said chromophore and said substrate.
 31. The method of claim 27further comprising the step of controlling the density of said firstspecies layer.
 32. The method of claim 27 wherein said attaching step isperformed by forming a covalent bond between electrophilic groups insaid chromophore and nucleophilic groups in said first species of saidfirst layer.
 33. The method of claim 27 wherein said chromophore is of amolecular weight ranging from 100 to 2000 atomic mass units.
 34. Themethod of claim 27 wherein said chromophore is a monomer.
 35. The methodof claim 34 wherein said chromophore is selected from the groupconsisting of Procion Red MX5B, Procion Brown MX-GRN, and Procion OrangeMX-2R.
 36. The method of claim 34 wherein said monomer is atrichloro-s-triazine covalently bonded to a dye.
 37. The method of claim36 wherein said dye is selected from the group consisting of MordantBrown 33, Acid Red 37, and Direct Orange
 31. 38. The method of claim 27wherein said first chromophore layer and said repeat chromophore layerexhibit noncentrosymmetry.
 39. The method of claim 27 further comprisingthe step of repeating steps c) and d) multiple times.
 40. The method ofclaim 27 wherein said first species in said first species layer ischemically or physically different from said first species in saidrepeat first species layer.
 41. The method of claim 40 wherein saidfirst species in at least two of said repeat first species layers arechemically or physically different from one another.
 42. The method ofclaim 27 wherein said chromophore in said first chromophore layer ischemically or physically different from said chromophore in said repeatchromophore layer.
 43. The method of claim 42 wherein said chromophorein at least two of said repeat chromophore layers are chemically orphysically different from one another.
 44. The method of claim 30wherein each of said attaching and adsorbing steps are accomplished bysubmersion of said substrate into, respectively, a volume of said firstspecies and a volume of said chromophore.
 45. The method of claim 30wherein each of said attaching and adsorbing steps are accomplished bywashing a solution of said first species or said chromophore over saidsubstrate.
 46. The method of claim 30 further comprising the step ofremoving said substrate after steps a)-d) are completed.
 47. The methodof claim 34 wherein said monomeric chromophore comprises i) chemicalmoieties capable of covalently bonding to said first species layer orsaid repeat first species layer; and ii) chemical moieties capable ofbinding ionically to said first species layer or said repeat firstspecies layer.
 48. The method of claim 47 wherein said chemical moietiescapable of covalently bonding comprise one or more triazine rings. 49.The method of claim 48 wherein said chemical moieties capable of bindingionically are ionizable groups selected from the group consisting ofsulfonate, phosphate, carboxylate, and quaternary ammonium.